1. Field of the invention
The present invention relates to aromatic hydroxy compounds or polymers substituted with acetal group(s) and a negative photoresist compositions containing the same.
2. Description of the Prior Art
The high integration of semiconductor devices has always been followed by significant advance in lithography. For example, ultra-fine patterns as small as sub-microns or quarter-microns in size must be required for the fabrication of ultra-LSI. Accordingly, the light sources used to form the fine patterns become shorter in wavelength from g-line to i-line and deep uv light, further to excimer laser, such as KrF laser, and finally to electron beams.
Since the photoresists prepared from conventional novolak-quinone diazides used for g-line or i-line show large absorption peaks at the wavelength range of deep uv light and excimer laser, a fine pattern cannot be obtained from the photoresists. Thus, there was a strong demand for a material which little absorbs the light belonging to such wavelength ranges. In response to the demand, active research has been directed to the development of chemical amplified photoresists based on polyhydroxystyrene derivatives which are smaller in absorption than novolak-quinonediazides.
Chemical amplified photoresists consist mainly of a base resin and a compound which generates acid upon radiation (hereinafter referred to as "photoacid generator"). Depending whether the photoacid acts to increase or decrease the solubility of the base resin, the chemical amplified photoresist is classified into positive or negative resist. The former is one which increase in solubility, the latter decreasing in solubility.
For example, U.S. Pat. No. 4,491,628 discloses a positive photoresist employing as a base resin polyhydroxystyrene whose hydroxy groups are partly replaced with t-butylcarboxy groups. A negative photoresist employs polyhydroxystyrene in combination with hexamethoxymethylmelamine as a crosslinking agent, as disclosed in Proc. SPIE, 1466, 246, 1991.
However, there is a significant disadvantage in such conventional positive and negative resists each: for positive resist, patterns are developed into a T shape by the amine in the air: for negative resist, as the methanol which is produced as a by-product upon crosslinking is escaped from the patterns formed, they undergo a considerable change in volume.